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Abstract:

A method of fabricating a light emitting diode using an epitaxial
lift-off process includes forming a sacrificial layer on a substrate,
forming a light emitting diode structure on the sacrificial layer with an
epitaxial material, forming a light reflecting layer on the light
emitting diode structure, and removing the sacrificial layer using an
etching process to separate the substrate from the light emitting diode
structure.

Claims:

1. A method of fabricating a thin film light emitting diode using an
epitaxial lift-off process, the method comprising: forming a sacrificial
layer on a substrate; forming a light emitting diode structure on the
sacrificial layer by epitaxial deposition; forming a light reflecting
layer on the light emitting diode structure; and removing the sacrificial
layer from the substrate using an etching process to separate the
substrate from the light emitting diode structure.

2. The method of claim 1, wherein forming the light emitting diode
structure on the sacrificial layer by epitaxial deposition includes:
forming a first contact layer on the sacrificial layer; forming a first
cladding layer on the first contact layer; forming a multiple quantum
well active layer on the first cladding layer; forming a second cladding
layer on the multiple quantum well active layer; and forming a second
contact layer on the second cladding layer.

3. The method of claim 1, further comprising attaching a handle to the
light-reflecting layer prior to removing the sacrificial layer.

4. The method of claim 1, further comprising dicing the light emitting
diode structure and the light reflecting layer subsequent to removing the
sacrificial layer to form a plurality of light emitting diodes.

5. The method of claim 1, further comprising, after separating the
substrate from the light emitting diode structure, fabricating one or
more additional thin film light emitting diodes using the substrate.

7. The method of claim 1, wherein the substrate has a diameter in the
range of approximately 3 inches to approximately 12 inches.

8. A method of fabricating a thin film light emitting diode using an
epitaxial lift-off process, the method comprising: receiving a substrate
previously used to form a thin film light emitting diode using an
epitaxial lift-off process; forming a sacrificial layer on the substrate;
forming a light emitting diode structure on the sacrificial layer by
epitaxial deposition; forming a light reflecting layer on the light
emitting diode structure; and removing the sacrificial layer from the
substrate using an etching process to separate the substrate from the
light emitting diode structure.

9. A thin film III-V semiconductor light emitting diode free of a
substrate, the light emitting diode comprising: a first contact layer; a
first cladding layer formed over the first contact layer; a multiple
quantum well active layer formed over the first cladding layer; a second
cladding layer formed over the multiple quantum well active layer; a
second contact layer formed over the second cladding layer; and a light
reflecting layer formed over the second contact layer.

10. The light emitting diode of claim 9, wherein the light-reflecting
layer includes a metallic layer.

11. The light emitting diode of claim 9, wherein the light-reflecting
layer includes at least one layer of a dielectric material.

12. The light emitting diode of claim 9, further comprising a handle
coupled to the light-reflecting layer.

13. The light emitting diode of claim 12, wherein the handle is in the
range approximately 5 μm to approximately 50 μm thick.

14. The light emitting diode of claim 12, wherein the handle is
permanently coupled to the light reflecting layer.

15. The light emitting diode of claim 12, wherein the handle is
temporarily coupled to the light reflecting layer.

16. A III-V semiconductor stack for forming a thin film light emitting
diode using epitaxial lift-off, the stack comprising: a substrate; a
sacrificial layer formed over the substrate; an LED structure formed over
the sacrificial layer; a light reflecting layer formed over the LED
structure; and a handle attached to the light reflecting layer.

17. The III-V semiconductor stack of claim 16, wherein the
light-reflecting layer includes a metallic layer.

18. The III-V semiconductor stack of claim 16, wherein the substrate has
a diameter in the range of between approximately 3 inches and
approximately 12 inches.

19. The III-V semiconductor stack of claim 16, wherein the LED structure
comprises: a first contact layer formed over the sacrificial layer; a
first cladding layer formed over the first contact layer; a multiple
quantum well active layer formed over the first cladding layer; a second
cladding layer formed over the multiple quantum well active layer; and a
second contact layer formed over the second cladding layer, wherein the
first contact layer and the second contact layer are more heavily doped
than the first cladding layer and the second cladding layer.

[0003] Embodiments of the invention relate generally to semiconductor
light emitting diodes (LEDs), and more particularly, to a LED fabricated
on a substrate, where the substrate is subsequently removed from the LED
in a non-destructive manner using an epitaxial lift-off (ELO) technique.

[0004] 2. Description of Related Art

[0005] III-V semiconductor devices are conventionally formed by first
growing the active layers of the device, comprising a variety of
materials in various combinations, on a bulk semiconductor substrate by
metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy
(MBE). The substrate provides a crystal template (e.g., GaAs or InP)
consisting of a highly periodic arrangement of atoms on which the active
layers are grown. The substrate does not contribute to the operation of
the device. The substrate either remains as part of the final device or
is removed during fabrication, leaving the active layers attached to some
other structure, or handle, which provides at least partial mechanical
stability. Most commonly, the substrate is removed through a combination
of mechanical grinding and chemical etching. The substrate is thus
effectively removed, but destroyed in the process.

SUMMARY

[0006] Example embodiments described herein include, but are not limited
to, methods for fabricating thin film light emitting diode structures
using epitaxial lift-off, and thin film light emitting diode structures
produced using epitaxial lift-off.

[0007] An embodiment of a method of fabricating a thin film light emitting
diode using an epitaxial lift-off process includes forming a sacrificial
layer on a substrate and forming a light emitting diode structure on the
sacrificial layer by epitaxial deposition. The method also includes
forming a light reflecting layer on the light emitting diode structure.
The method further includes removing the sacrificial layer from the
substrate using an etching process to separate the substrate from the
light emitting diode structure.

[0008] In some embodiments, the method further includes forming a first
contact layer on the sacrificial layer. The method may further include
forming a first cladding layer on the first contact layer, forming a
multiple quantum well active layer on the first cladding layer, and
forming a second cladding layer on the multiple quantum well active
layer. The method may also include forming a second contact layer on the
second cladding layer.

[0009] In some embodiments, the method also includes attaching a handle to
the light-reflecting layer prior to removing the sacrificial layer.

[0010] In some embodiments, the method also includes dicing the light
emitting diode structure and the light reflecting layer subsequent to
removing the sacrificial layer to form a plurality of light emitting
diodes.

[0011] In some embodiments, forming the light emitting diode structure
includes forming a III-V semiconductor light emitting diode structure. In
some embodiments, the substrate has a diameter in the range of
approximately 3 inches to approximately 12 inches.

[0012] In some embodiments, after fabrication of a first light emitting
diode, the method further includes fabricating one or more additional
thin film light emitting diodes using the substrate. In some embodiments,
the method includes receiving a substrate previously used to form a thin
film light emitting diode using an epitaxial lift-off process, and using
the substrate to form an additional thin film light emitting diode.

[0013] Another embodiment is a thin film III-V semiconductor light
emitting diode free of a substrate, the light emitting diode includes a
first contact layer and a first cladding layer formed over the first
contact layer. The light emitting diode further includes a multiple
quantum well active layer formed over the first cladding layer. The light
emitting diode also includes a second cladding layer formed over the
multiple quantum well active layer, a second contact layer formed over
the second cladding layer, and a light reflecting layer formed over the
second contact layer.

[0014] In some embodiments, the light-reflecting layer includes a metallic
layer. In some embodiments, the light-reflecting layer includes at least
one layer of dielectric material.

[0015] In some embodiments, the light emitting diode further includes a
handle coupled to the light-reflecting layer. In some embodiments, the
handle includes a metal, a polymer, or both. In some embodiments, the
handle is in the range of approximately 5 μm to approximately 50 μm
thick.

[0016] In some embodiments, the handle is permanently coupled to the light
reflecting layer. In some embodiments, the handle is temporarily coupled
to the light reflecting layer.

[0017] Another embodiment includes a III-V semiconductor stack for forming
a thin film light emitting diode using epitaxial lift-off. The stack
includes a substrate and a sacrificial layer formed over the substrate.
The stack also includes an LED structure formed over the sacrificial
layer, a light reflecting layer formed over the LED structure, and a
handle attached to the light reflecting layer.

[0018] In some embodiments, the light-reflecting layer includes a metallic
layer. In some embodiments, the substrate has a diameter in the range of
between approximately 3 inches and approximately 12 inches. In some
embodiments, the LED structure includes a first contact layer formed over
the sacrificial layer, a first cladding layer formed over the first
contact layer, and a multiple quantum well active layer formed over the
first cladding layer. The LED structure also includes a second cladding
layer formed over the multiple quantum well active layer, and a second
contact layer formed over the second cladding layer. In some embodiments,
the first contact layer and the second contact layer are more heavily
doped than the first cladding layer and the second cladding layer.

[0019] In some embodiments, the sacrificial layer includes an AlGaAs
material.

[0020] The summary above is provided merely to introduce a selection of
concepts that are further described below in the detailed description.
The summary is not intended to identify key or essential features of the
claimed subject matter, nor is it intended to be used as an aid in
limiting the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] These figures are intended to illustrate the embodiments taught
herein and are not intended to show relative sizes and dimensions (e.g.,
they are not drawn to scale), or to limit the scope of examples or
embodiments. In the drawings, each identical or nearly identical
component that is illustrated in various figures is represented by a like
numeral.

[0022] FIG. 1 depicts a diagrammatic representation of one example of a
device structure prior to an epitaxial lift-off process in accordance
with one embodiment;

[0023]FIG. 2 depicts a diagrammatic representation of one example of a
device structure on a handle, and a separate substrate, after an
epitaxial lift-off process in accordance with one embodiment;

[0024]FIG. 3 depicts a flow diagram of several examples of a process of
fabricating a light emitting diode in accordance with one embodiment; and

[0025]FIG. 4 depicts a flow diagram of one example of a process for
forming a light emitting diode structure in accordance with one
embodiment.

DETAILED DESCRIPTION

[0026] Exemplary embodiments use epitaxial lift-off (ELO) techniques
during fabrication of light-emitting diodes (LEDs), and in particular,
thin film LEDs free of a substrate. As discussed above, conventional
fabrication techniques can destroy the substrate after the device is
formed or leave the substrate in place. By contrast, an epitaxial
lift-off process, in accordance with some embodiments, provides a
non-destructive way of removing the substrate from a wafer of LED
structures after the LED structures are formed on the wafer such that the
substrate can be reused for fabricating more LEDs. For example, in some
embodiments, a 4-inch wafer may accommodate between 5,000 and 20,000 LED
structures. In some embodiments, a 4-inch wafer may accommodate more than
20,000 LED structures. A chemical etch can be used to remove a
sacrificial release layer grown between the substrate and epitaxial
layers, resulting in separation of the epitaxial layers from the
substrate. The substrate remains intact and, after a re-polishing step,
may be used again as the crystal template for another growth run. In some
embodiments, the substrate may be reused up to ten times for forming
additional epitaxial layers thereon. In some embodiments, the substrate
may be reused more than ten times for forming additional thin films of
LED structures. The ability to remove the epitaxially-grown layers while
maintaining the integrity of the substrate is unique to the ELO process
and is difficult or impossible to achieve with other known techniques.

[0027] In one embodiment, an LED fabrication process begins with
epitaxially growing one or more LED structures over an epitaxial
sacrificial release layer on a substrate. In some embodiments, the
release layer is 8-10 nanometers thick. In some embodiments, the
epitaxial sacrificial release layer is lattice-matched to the substrate
on which it is grown. The material for the sacrificial release layer has
an extremely high etch rate compared with the etch rates of the other
layers in the LED structure. For example, in some embodiments, a
sacrificial release layer for epitaxial growth on a GaAs substrate
includes high Al-content AlGaAs. A light reflecting layer and a handle
may be formed over the LED structures. After the layers and the handle
are formed, the release layer is removed by an etching process, which
separates the wafer of LED structures from the substrate in a manner that
does not damage the substrate or the LED structures. Tensile strain
provided by the handle and/or the light reflecting layer enables lift-off
of the thin film layers without any external mechanical intervention: it
is not necessary to add weights to the LED structure or to add Kapton or
wax over the substrate or release layer for lift-off.

[0028] Each LED structure comprises transparent n- and p-type cladding
layers surrounding an undoped multiple quantum well active layer, which
is designed to emit light at the design wavelength of the LED. One or
more light reflecting layers are deposited on the top of the wafer, which
forms the back of the LED. In some embodiments, each of the one or more
light reflecting layers a thickness between 10 nm and 1,000 nm. The
light-reflecting layer(s) may include, for example, a metallic layer
(e.g., silver) and/or multiple layers of dielectric materials. The
light-reflecting layer serves at least two purposes: it forms an optical
reflector to reflect light emitted by the device, as well as acting as an
adhesion layer between a handle and the epitaxial layers. In some
embodiments, the light-reflecting layer may provide at least some
strength to the LED structure. In embodiments including a handle layer,
the handle is applied to, or formed on, the light reflecting layer to
provide mechanical stability to the epitaxial layers after the ELO
process. In some embodiments, the handle may be formed of a material that
includes copper, another similar metal, or combination of metals. In some
embodiments, the handle may be formed of a material that includes a
polymer, a combination of polymers, or a combination of one or more metal
and one or more polymers. In some embodiments, the handle may have a
thickness in the range 5 μm to 50 μm.

[0029] FIG. 1 depicts a diagrammatic representation of one example of a
III-V semiconductor stack 95 for forming a light emitting diode device
100, according to one embodiment. The device 100 includes a light
emitting diode structure 110, a light reflecting layer 112 formed on the
LED structure 110, and a handle 114 attached to the light-reflecting
layer 112. The semiconductor stack 95 includes a substrate 116 that
supports a release layer 118 upon which the LED structure 110 is formed.

[0030] The LED structure 110 includes several epitaxial layers including a
multiple quantum well (MQW) active layer 120, a first cladding layer 122
formed on one side of the MQW active layer 120, a second cladding layer
124 formed on an opposite side of the MQW active layer 120 from the first
cladding layer 122, a top contact layer 126 formed on the first cladding
layer 122, and a back contact layer 128 formed on the second cladding
layer 124. In some embodiments, the top contact layer 126 is doped in a
range of 2×1018 (2E18) cm-3 to 5E18 cm -3 and the
back contact layer 128 is doped in a range of 2E18 cm-3 to 5E18
cm-3. The release layer 118 may be grown before any layers of the
LED structure 110 are grown and formed between the substrate 116 and the
top contact layer 126. The light reflecting layer 112 may be formed on
the back contact layer 128 and may be configured to reflect light
generated by the LED structure 110, and more particularly, light
generated by the MQW active layer 120. Any of the epitaxial layers, for
example, the LED structure 110, the release layer 118, the substrate 116,
and/or the light reflecting layer 112, may be formed directly on an
adjacent layer or indirectly over the adjacent layer with a buffer or
diode in between.

[0031] The handle 114 may be affixed to or formed on the light-reflecting
layer 112 to provide mechanical support for the LED structure 110 and the
light-reflecting layer 112 after the device 100 is separated from the
substrate 116. The handle 118 may be permanently or temporarily affixed
to the light-reflecting layer 112.

[0032] After all of the epitaxial layers are grown, a chemical etch is
used to remove the release layer 118, thus separating the epitaxial
layers of the device 100 from the substrate 116. In some embodiments, the
substrate 116 may be reused up to 10 times for fabricating additional
wafers.

[0033] After the release layer 118 is removed, the LED structure 110 can
be flexible because it is no longer mechanically supported by the
substrate 116. In some embodiments, the back contact layer 128, the light
reflecting layer 112 and/or the handle 114 have enough tensile strength
to provide mechanical support to the LED structure 110 such that the
integrity of the LED structure 110 is not compromised during handling
after the release layer is removed 118, which can allow for high
manufacturing yields. For example, some embodiments may result in a
manufacturing yield of at least 90% functional devices from the
deposition and lift-off process. In some embodiments, a manufacturing
yield of at least 95% functional devices from the deposition and lift-off
process may be attained.

[0034]FIG. 2 depicts a diagrammatic representation of one example of the
device 100 of FIG. 1, and the substrate 116, after an ELO process,
according to one embodiment. After the ELO process, the release layer 118
has been removed (e.g., by etching), exposing the top contact layer 126
of the device 100 and separating the device 100, which may include the
handle 114, from the substrate 116. The substrate 116 may be cleaned and
re-polished to prepare it for use in another growth run. The epitaxial
layers of the device 100 may be temporarily affixed to a carrier (not
shown), such as a silicon substrate, a glass substrate, a sapphire
substrate, or a metal substrate, using a temporary adhesive to allow
further processing of the device. The carrier is useful for providing
additional mechanical stability to the device because without the
carrier, the device 100 may be flexible with only the handle layer 114
providing mechanical stability. The epitaxial layers of the device 100
may then be processed using, for example, conventional lithographic
techniques to form multiple LED devices. After processing, the epitaxial
layers of the device 100 may be removed from the carrier and diced, cut
or otherwise divided into multiple individual devices, which may then be
individually packaged. In some embodiments, after dividing the device
into multiple individual devices, the individual devices may be from
approximately 50 μm to 1,000 μm in size.

[0035] In one embodiment, a LED fabricated using an ELO process, such as
described above with respect to FIGS. 1 and 2, emits light at a design
wavelength when connected to an external voltage bias. The light
emitting, multiple quantum well active layer 120 of the device may emit
light in all directions. The light-reflecting layer 112 provides a mirror
to reflect light emitted toward the back of the device and redirect it
out the front. The epitaxial layers of the device 100 are inverted by the
ELO process such that the device is grown with what will be the top or
light emitting surface (i.e., the top contact layer 126) of the device
formed at the bottom of the epitaxial layer stack, closest to the
substrate 116. The first cladding layer 122 and the second cladding layer
124 confine the carriers to the active layer (e.g., the MQW active layer
120) of the device 100.

[0036]FIG. 3 depicts a flow diagram of several examples of a process 300
of fabricating a light emitting diode, according to one embodiment. The
process 300 begins at step 302. At step 304, a sacrificial layer (e.g.,
release layer 118) is formed over a substrate (e.g., substrate 116). At
step 306, a light emitting diode structure (e.g., LED structure 110) is
formed over the sacrificial layer by epitaxial deposition. One example of
a process for forming the light emitting diode structure is described
below with respect to FIG. 4. At step 308, a light reflecting layer
(e.g., light reflecting layer 112) is formed over the light emitting
diode structure. At step 310, the sacrificial layer is removed from the
substrate using an etching process, which separates the substrate from
the light emitting diode structure. Process 300 ends at step 312. In one
embodiment, the process 300 can be used to fabricate a thin film LED
using one or more epitaxial materials and free of a substrate on a wafer
of about 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 inches in diameter, or on a
wafer having any diameter between about 3 and 12 inches.

[0037] In another embodiment, at step 314, a handle (e.g., handle 114) is
affixed to or formed over the light-reflecting layer prior to removing
the sacrificial layer at step 310. In yet another embodiment, at step
316, the substrate is polished and reused to fabricate another light
emitting diode. In this embodiment, process 300 then returns to step 302
to repeat the fabrication process.

[0038]FIG. 4 depicts a flow diagram of one example of a process for
forming a light emitting diode structure in accordance with one
embodiment, such as described in step 306 of FIG. 3.

[0039] The process for performing step 306 begins at step 402. At step
404, a first contact layer (e.g., top contact layer 126) is formed over
the sacrificial layer. At step 406, a first cladding layer (e.g.,
cladding 122) is formed over the first contact layer. At step 408, a MQW
active layer (e.g., MQW active layer 120) is formed over the first
cladding layer. At step 410, a second cladding layer (e.g., cladding 124)
is formed over the MQW active layer. At step 412, a second contact layer
(e.g., back contact layer 128) is formed over the second cladding layer.
The process for performing step 306 ends at step 414.

[0040] According to some embodiments, the ELO process can produce a device
that is comparable to devices fabricated using a conventional substrate
grinding/etching removal process. Because the substrate cost in III-V
semiconductor manufacturing can be a significant component of the overall
cost, the ability to reuse the substrate multiple times can provide
significant cost savings. By using ELO, LEDs can be manufactured at a
lower cost than possible with other manufacturing techniques through the
reuse of the substrate. Further, the ELO process, according to some
embodiments, can permit high manufacturing yields. Material defects
resulting from the epitaxial growth, or from the ELO process, affect only
the device at the location where the defect occurs. Therefore, other
devices, including adjacent ones, are unaffected by such defects.

[0041] Another advantage of the ELO process is that it results in the
complete removal of the substrate. In some embodiments, this allows the
ELO LED to be mounted directly on a heat sink without an intervening
substrate layer. Accordingly, the absence of the substrate will result in
the LED operating at a lower temperature than it would if the substrate
were present. Lowering the operating temperature increases LED efficiency
and increases LED lifetime.

[0042] Having thus described several exemplary embodiments of the
invention, it is to be appreciated that various alterations,
modifications, and improvements will readily occur to those skilled in
the art. For example, in some embodiments, the ELO process may be used to
fabricate LED structures on large size substrates (e.g., between
approximately 3- and 12-inches in diameter). In some embodiments, the ELO
process may be used to fabricate a variety of LEDs, from small,
low-brightness LEDs to large, high-brightness LEDs. Such alterations,
modifications, and improvements are intended to be part of this
disclosure, and are intended to be within the scope of the invention.
Accordingly, the foregoing description and drawings are by way of example
only.